The present invention relates to a solid-state image sensor for obtaining a two-dimensional image using a photoelectric conversion effect.
Prior art will be described with reference to the drawings.
FIG. 1 is an example of the circuit diagram of a solid-state image element called "an amplifying MOS sensor".
In FIG. 1, unit cells 3.times.3 consisting of amplifying transistors 2-1-1, 2-1-2, . . . , 2-3-3 for reading signals of photodiodes 1-1-1, 1-1-2, . . . , 1-3-3, vertical select transistors 3-1-1, 3-1-2, . . . , 3-3-3 for selecting lines for reading signals and reset transistors 4-1-1, 4-1-2, . . . , 4-3-3 for resetting signal charges are arranged two-dimensionally.
It is noted that more unit cells are arranged in the actual device. Horizontal address lines 6-1, . . . , 6-3 connected to a vertical shift register 5 in the horizontal direction are connected to gates of vertical select transistors 3-1-1, 3-1-2, . . . , 3-3-3, respectively, to determine a line for reading a signal. Reset lines 7-1, . . . , 7-3 are connected to gates of reset transistors 4-1-1, 4-1-2, . . . , 4-3-3, respectively. Sources of the amplifying transistors 2-1-1, 2-1-2, . . . , 2-3-3 are connected to vertical signal lines 8-1, . . . , 8-3, respectively and load transistors 9-1, . . . , 9-3 are provided on one ends of the sources thereof, respectively. Other ends of the vertical signal lines 8-1, . . . , 8-3 are connected to a horizontal signal line 11 through horizontal select transistors 19-1, . . . , 19-3 selected by a select pulse supplied from a horizontal shift register 10, respectively.
FIG. 2 is an example of the sectional structure of a prior art photodiode.
In FIG. 2, a reference numeral 20 denotes a P-type semiconductor substrate having uniform impurity concentration (about 1.times.10.sup.15 cm.sup.-3), a reference numeral 21 denotes a P-well formed by injecting ions of P-type impurities such as boron (B) (with a concentration of about 1.times.10.sup.17 cm.sup.-3), a reference numeral 22 denotes an N-type region formed by injecting ions of N-type impurities such as phosphorous (P) and reference numeral 23 denotes a depletion region at a PN junction.
In the structure shown in FIG. 2, the concentration of the P-type semiconductor substrate 20 is low (i.e., high resistance) and the concentration of the P-well 21 is higher than that of the P-type semiconductor substrate 20. With such a structure, diffusion current from the P-type semiconductor substrate 20 is high and a large amount of current is generated in the depletion region 23 since crystal defects are introduced into the P-well 21 as a result of ion implantation. This causes a problem that diode dark current which is the sum of the diffusion current and the generation current is high. The prior art structure also has a problem that, due to the above reason, photo-sensitivity is low and that the phenomenon of the leakage of signal charges into adjacent photodiodes (color crosstalk) increases.
The amplifying MOS image sensor shown in FIG. 1 which has amplifying transistors (2-1-1, . . . , 2-3-3) within a unit pixel, has characteristically high sensitivity. On the other hand, it has a disadvantage in that non-uniform gate threshold voltages appear as fixed pattern noise. To get rid of that disadvantage, there is known a method of providing noise cancelers 18-1, . . . , 18-3 on ends of vertical signal lines, respectively.
FIG. 3 shows a circuit diagram wherein, the noise cancelers 18-1, . . . , 18-3 are provided. FIG. 4 shows a specific example of the noise canceler. The noise canceler 18 mainly comprises, for example, a clamp capacitance C1, a clamp transistor Tr1 for resetting a node connected to the clamp capacitance C1, a sample hold capacitance C2, a sample hold transistor Tr2 for reading a signal from the node connected to the clamp capacitance C1. The differential signal between a dark period and a bright period (light incidence period) is outputted to the vertical signal line 11 by the nose canceler. The noise cancel operation of the noise canceler is conducted for a horizontal blanking period or a short period of time such as 10.9 microseconds in a NTSC system and 3.77 microseconds in a High-Vision system. This requires therefore the transistors and capacitance which constitute the noise canceler 18 to have high-speed operation performance.
If a transistor is formed within the P-well shown in FIG. 2, the response speed of the transistor is determined by the product of the resistance of the P-well and the capacitance of the depletion layer of the source-drain region of the transistor due to the high resistance of the substrate. This is because current is supplied from a well contact formed on the P-well to the source-drain region in response to a variation in the potential of the transistor. In this case, the P-well may well have quite high concentration (or low resistance) to fasten the response speed. If so, however, it is difficult to control threshold voltage by ion implantation into the channel region of the transistor. The clamp capacitance C1 and the sample hold capacitance C2 shown in FIG. 4 are formed on an insulating film formed on the semiconductor substrate. The capacitance of the insulating film is, therefore, added to the capacitance C1 and C2 in series or in parallel. The response speed of capacitance of the insulating film is determined by the product of the P-well resistance and capacitance. For the same reason of the above-stated transistor, it is difficult to fasten the response speed.
Therefore, it is difficult to realize the high-speed operation of the transistor and the capacity which composes a noise canceler as far as it used the conventional wafer section structure shown in FIG. 2.
As described above, the prior art MOS-type solid-state image sensor has disadvantages in that dark current at the photoelectric conversion portion is high and that component noise during a dark period is large. It also has disadvantages of low sensitivity, color crosstalk and/or high degree of blooming. "Blooming" here is a phenomenon that signal charges are poured into adjacent pixels if intensifier light is incident. Moreover, the prior art MOS-type solid-state image sensor has a disadvantage in that it is difficult to realize the high-speed operation of the transistors which are the constituents of the noise canceler.